This thesis deals with a partial discharge(PD) measurement techniques to apply a PD test to low-voltage induction motors as a part of quality assurance. The objective of this study is to develop a PD measurement system, which can detect charge below 1PC, to prevent insulation degradation of the test sample from the test voltage.
PD pulses are obtained by a straight detection principle considering the wide use in fields. The fabricated PD measurement system is composed of a coupling network with a discharge free capacitor and a detection impedance, a low-noise amplifier, and an associated electronics. The low cutoff frequency of the detection circuit is set at 1MHz(-3dB) to attenuate AC voltage by 270dB and to pass discharge pulses only without any distortion.
A shielding enclosure of Ni-Cu double structure and a high voltage filter having a high cutoff frequency of 3.5kHz were designed to get a better measurement condition against radiation and conduction noises from outside. The PD measurement system was tested to noise intereference and the peak noise level appeared 3mVp-p.
Calibrations of the PD measurement system were carried out according to the IEC 60270 and the calculated sensitivities by a standard calibrator on low-voltage induction motors were 22.3mV/pC for 1φ 0.5HP and 11.4mV/pC for 3φ 5HP, respectively. Also, the sensitivity to the positive and negative polarity pulses were same in ranges from 1pC to 50pC.
In the PD test on the motors, it was confirmed that the polarity of PD pulses changes depending on the shape of a void or a interface, and an apparent charge of 0.7pC was measured at AC 664Vpeak.